194 PRINCIPLES OF ANIMAL NUTRITION. 
2. The oxidation somewhere in the organism of the immediate 
products of this decomposition to the final excretory products. 
3. Since the state of contraction appears to be only an exagger- 
ation of the muscular condition during rest, we may reasonably 
suppose that there is a continual re-formation of the “contractile 
substance” going on. 
4. As secondary effects there is a marked increase in the activ- 
ity of circulation and respiration, thus involving supplementary 
muscular exertion. 
It is plain that however interesting and important to the physi- 
ologist may be studies of the changes in the muscle itself, from 
the point of view of the statistics of nutrition the important thing 
is the total effect upon the expenditure of matter and energy by the 
organism under varying conditions of work. The energy relations 
of the subject will be discussed stiibsequently in Part II. Here we 
are concerned more particularly with the nature of the material 
expended in the production of work, and as a matter of convenience 
we may, as in the two preceding chapters, take up first the effect. 
upon the proteid metabolism and second that upon the metabolism 
of the non-nitrogenous substances. 
Effects wpon Proteid Metabolism. 
Ear.ier INVESTIGATIONS.—Since the muscles, which are the 
instruments by means of which work is produced, are composed 
essentially of proteid material, it was natural to regard the proteids 
as the source of muscular power and to assume that the energy 
developed during work was supplied by an increased metabolism 
of these substances. This view was supported by the authority of 
Liebig, who, however, does not appear to have based it upon any 
actual experiments, and it was quite generally, although not 
universally, accepted. 
C. Voit * appears to have been the first to subject this idea to 
investigation. His experiments were made upon a dog weighing 
about 32 kilograms. The work performed, by running in a tread- 
mill, was considerable, being estimated to average 1.7 kgm. per 
second for the whole twenty-four hours. Experiments were made 
* Untersuchungen tiber den Einfluss des Kochsalzes, des Wassers, und 
der Muskelbewegungen auf den Stoffwechsel. 1860. Compare the summary 
by E. v. Wolff in Die Ernahrung der landw. Nutzthiere, pp. 386-388. 
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